Transcript Heme and Copper Oxygenases and Oxidases

Organometallic Chemistry
JHU Course 030.442
Prof. Kenneth D. Karlin
Spring, 2009
Kenneth D. Karlin
Department of Chemistry, Johns Hopkins University
[email protected]
http://www.jhu.edu/~chem/karlin/
Organometallic Chemistry
p. 1
030.442
Prof. Kenneth D. Karlin
Spring, 2009
Class Meetings: TTh, 12:00 – 1:15 pm
Textbook – The Organometallic Chemistry of the Transition Metals”
4th Ed., R. H. Crabtree
Course Construction: Homeworks, Midterm Exams (1 or 2), Oral Presentations
Rough Syllabus Most or all of these topics
• Introduction, History of the field
• Reaction Types
Oxidative Addition
• Transition Metals, d-electrons
Reductive elimination
–
• Bonding, 18 e Rule (EAN Rule)
Insertion – Elimination
Nucleophilic/electrophilic Rxs.
• Ligand Types / Complexes
• Types of Compounds
• Catalysis – Processes
Wacker oxidation
M-carbonyls, M-alkyls/hydrides
Monsanto acetic acid synthesis
M-olefins/arenes
Hydroformylation
M-carbenes (alkylidenes alkylidynes) Polymerization- Olefin metathesis
Water gas-shift reaction
Other
Fischer-Tropsch reaction
p. 2
p. 3
Reaction Examples
• Oxidative Addition
Reductive Elimination
Vaska’s complex
• Carbonyl Migratory Insertion
CH3Mn(CO)5
CO
O
CH3CMn(CO)5
• Reaction of Coordinated Ligands
O
(Iron pentacarbonyl)
(CO)4Fe–C O + :OH– ––––> (CO)4Fe
––––––>
(CO)4Fe–H
+
CO2
O
H
Reaction Examples - continued
p. 4
• Wacker Oxidation
C2H4 (ethylene) + ½ O2 –––> CH3CH(O) (acetaldehyde)
Pd catalyst, Cu (co-catalyst)
• Monsanto Acetic Acid Synthesis
CH3OH (methanol) + CO –––> CH3C(O)OH (acetic acid) (Rh catalyst)
• Ziegler-Natta catalysts – Stereoregular polymerization of 1-alkenes (a-olefins)
1963 Nobel Prize
n CH2=CHR
–––>
–[CH2-CHR]n–
Catalyst: Ti compounds and organometalllic Al compound (e.g.,
(C2H5)3Al )
• Olefin metathesis – variety of metal complexes
2005 Nobel Prize – Yves Chauvin, Robert H. Grubbs, Richard R. Schrock
p. 5
Organo-transition Metal Chemistry History-Timeline
• Main-group Organometallics
1760 - Cacodyl – tetramethyldiarsine,
from Co-mineral with arsenic
1899 –> 1912 Nobel Prize: Grignard reagents (RMgX)
n-Butyl-lithium
• 1827 – “Zeise’s salt” - K+ [(C2H4)PtCl3]–
Synthesis: PtCl4 + PtCl2 in EtOH, reflux, add KCl
Bonding- Dewar-Chatt-Duncanson model
p. 6
Organo-transition Metal Chemistry History-Timeline (cont.)
1863 - 1st metal-carbonyl, [PtCl2(CO)2]
1890 – L. Mond, (impure) Ni + xs CO –––> Ni(CO)4 (highly toxic)
1900 – M catalysts; organic hydrogenation (---> food industry, margerine)
1930 – Lithium cuprates, Gilman regent, formally R2Cu–Li+
1951 – Ferrocene discovered. 1952 -- Sandwich structure proposed
(Cp)2Fe
Cp = cyclopentadienyl anion)
(h5-C5H5)2Fe
(pentahapto)
Solid-state
structure
Ferrocene was first prepared unintentionally. Pauson and Kealy, cyclopentadieny-MgBr and
FeCl3 (goal was to prepare fulvalene) But, they obtained a light orange powder of "remarkable
stability.”, later accorded to the aromatic character of Cp– groups. The sandwich compound structure
was described later; this led to new metallocenes chemistry (1973 Nobel prize, Wilkinson & Fischer).
The Fe atom is assigned to the +2 oxidation state (Mössbauer spectroscopy).
The bonding nature in (Cp)2Fe allows the Cp rings to freely rotate, as observed by NMR
spectroscopy and Scanning Tunneling Microscopy. ----> Fluxional behavior. (Note: Fe-C bond
distances are 2.04 Å).
p. 7
Organo-transition Metal Chemistry History-Timeline (cont.)
1955 - Cotton and Wilkinson (of the Text) discover organometallic-complex
fluxional behavior (stereochemical non-rigidity)
The capability of a molecule to undergo fast and reversible intramolecular isomerization, the energy
barrier to which is lower than that allowing for the preparative isolation of the individual isomers at
room temperature. It is conventional to assign to the stereochemically non-rigid systems those
compounds whose molecules rearrange rapidly enough to influence NMR line shapes at
temperatures within the practical range (from –100 °C to +200 °C ) of experimentation. The
energy barriers to thus defined rearrangements fall into the range of 5-20 kcal/mol (21-85 kJ/mol).
Aside:
Oxidation State
18-electron Rule
p. 8
Fluxional behavior; stereochemical non-rigidity (cont.)
Butadiene iron-tricarbonyl
Xray- 2 CO’s equiv, one diff., If retained in solution, expect,
2:1 for 13-C NMR. But, see only 1 peak at RT. Cooling
causes a change to the 2:1 ratio expected.
Two possible explanations:
(1)Dissociation and re-association or (2) rotation of
the Fe(CO)3 moiety so that CO’s become equiv.
Former seems not right, because for example addition
of PPh3 does NOT result in substitution to give
(diene)M(CO)2PPh3.
Note: You can substitute PPh3 for CO, but that requires
either high T or hv. So, the equivalency of the CO groups
is due to rotation without bond rupture, pseudorotation.
13C-NMR
spectra
CO region, only
p. 9
Berry Pseudorotation
Pseudorotation: Ligands 2 and 3 move from axial to equatorial
positions in the trigonal bipyramid whilst ligands 4 and 5 move from
equatorial to axial positions. Ligand 1 does not move and acts as a
pivot.
At the midway point (transition state) ligands 2,3,4,5 are
equivalent, forming the base of a square pyramid. The motion is
equivalent to a 90° rotation about the M-L1 axis. Molecular
examples could be PF5 or Fe(CO)5.
p. 10
The Berry mechanism, or Berry pseudorotation mechanism, is a type of
vibration causing molecules of certain geometries to isomerize by exchanging the
two axial ligands for two of the equatorial ones. It is the most widely accepted
mechanism for pseudorotation. It most commonly occurs in trigonal bipyramidal
molecules, such as PF5, though it can also occur in molecules with a square
pyramidal geometry.
The process of pseudorotation occurs when the two axial ligands close like a
pair of scissors pushing their way in between two of the equatorial groups which
scissor out to accommodate them. This forms a square based pyramid where the
base is the four interchanging ligands and the tip is the pivot ligand, which has not
moved. The two originally equatorial ligands then open out until they are 180
degrees apart, becoming axial groups perpendicular to where the axial groups
were before the pseudorotation.
Organo-transition Metal Chemistry History-Timeline (cont.)
p. 11
1961 – D. Hodgkin, X-ray structure – Coenzyme Vitamin B12 (see other page)
Oldest organometallic complex (because biological) (see other
page)
R H
H R
Catalysis
C C
C C
of 1,2-shifts
H H
H H
(mutases)
or
(homocysteine) RSH
Homocysteine
methylation
[B12CoIII-CH3]+
Methylmalonyl-CoA
––> Succinyl-CoA
(CoA = coenzyme A)
RSCH3 (methionine)
[B12CoI]–
1963 - Ziegler/Natta Nobel Prize, polymerization catalysts
1964 - Fischer, 1st Metal-carbene complex
1965 – Cyclobutadieneiron tricarbonyl, (C4H4)Fe(CO)3
– theory before experiment
(C4H4) is anti-aromatic (4 p-electrons)
With -Fe(CO)3,
C4H4 behaves as aromatic
1965 – Wilkinson hydrogenation catalyst, Rh(PPh3)3Cl
p.12
V
Vitamin B-12 is a water
soluble vitamin, one of the eight B vitamins. It is normally involved in
i
the metabolism of every cell of the body, especially affecting DNA synthesis and regulation, but
also fatty acid synthesis
and energy production.
t
Vitamin B-12 is the name for a class of chemically-related compounds, all of which have
vitamin activity. It is structurally
the most complicated vitamin. A common synthetic form of the
a
vitamin, cyanocobalamin (R = CN), does not occur in nature, but is used in many
pharmaceuticals, supplements
and as food additive, due to its stability and lower cost. In the
m
body it is converted to the physiological forms, methylcobalamin (R = CH ) and
adenosylcobalamin, leaving
i behind the cyanide.
5-deoxyadenosyl group
n
B
1
2
C
o
e
3
p. 13
Organo-transition Metal Chemistry History-Timeline (cont.)
1973 – Commercial synthesis of L-Dopa (Parkinson’s drug)
asymmetric catalytic hydrogenation
2001 Nobel Prize – catalytic asymmetric synthesis, W. S. Knowles (Monsanto
Co.)
R. Noyori,, (Nagoya, Japan), K. B. Sharpless
(Scripps, USA)
1982, 1983 – Saturated hydrocarbon oxidative addition, including methane
1983 – Agostic interactions (structures)
p. 14
AGOSTIC INTERACTIONS:
Agostic – derived from Greek word for "to hold on to oneself”
C-H bond on a ligand that undergoes an interaction with the metal
complex resembles the transition state of an oxidative addition or
reductive elimination reaction.
Detected by NMR spectroscopy, X-ray diffraction
Compound above: Mo–H = 2.1 angstroms, IR bands were observed at
2704 and 2664 cm–1 and the agostic proton was observed at –3.8 ppm.
The two hydrogens on the agostic methylene are rapidly switching
between terminal and agostic on the NMR time scale.
p. 15
Organometallic Chemistry
Definition: Definition of an organometallic compound
Anything with M–R bond R = C, H (hydride)
Metal (of course) Periodic Table – down & left
electropositive element (easily loses electrons)
NOT:
• Complex which binds ligands via, N, O, S, other
M-carboxylates, ethylenediamine, water
• M–X where complex has organometallic behavior, reactivity patterns
e.g., low-valent
Oxidation State
M
–N
R'
R''
Charge left on central metal as the ligands are removed in their ‘usual’
closed shell configuration (examples to follow).
dn
for compounds of transition elements
N d < (N+1) s or (N+1) p in compounds
e.g., 3 d < 4 s or 4 p
d
d
n computation – very important in transition metal chemistry
n
zero oxidation state of M in M-complex has a
configuration d n where n is the group #.
Examples: Mo(CO)6
Mo(0) d
n
= d 6 (CO, neutral)
HCo(CO)4 H is hydride, H–, --> --> Co(I), d n = d 8
Group 5
Group 6
Group 7
V(CO)6–
Cr(CO)6
Mn(CO)6+
V(–1)
Cr(0)
Mn(+1)
d6
d6
d6
Isoelectronic and isostructural compounds (importance of d n)
Effective Atomic # Rule; 18-Electron Rule (Noble gas formalism)
# of electrons in next inert gas =
# Metal valence electrons + s (sigma) electrons from ligands
Rule: For diamagnetic (spin-paired) mononuclear complexes in
organotransition metal compounds, one never exceeds the E.A.N.
p. 16
Cr(CO)6
(CO)6
d6
Cr --->
6 electrons
pairs from 6 ligands 12 electrons
––> to [Ar] configuration
18 electrons
e– -
p. 17
(will see more in M.O. diagram)
Consequence of EAN Rule:
leads to prediction of maximum in coordination #
Max coordination # = (18 – n) / 2 n is from d n
dn
10
8
6
4
2
0
Max Coord # 4
5
6
7
8
9
.
– Change in 2-electrons results in change of only one in Coord. #
– Any Coord. # less than Max # ---> “coordinatively unsaturated”
Fe(CO)42–
e–
18
Fe(–2)
d 10
4-coord
–2e– +CO
2e–
–CO
Fe(CO)5
18 e–
Fe(0)
d8
5-coord
both Coord. Saturated
p. 18
[ReH9]2–
e.g., as Ba2+ salt
Re(VII), (Mn,Tc, Re triad)
d 0, 9 hydride ligands; CN = 9
Geometry: Face capped trigonal prism
A compound not obeying an rules
Fe5(CO)15C
Iron-carbonyl carbide
p. 19
Eighteen-Electron Rule - Examples
Co(NH3)63+
Cr(CO)6
Obey 18-electron rule for different reasons
Carbonyl Compounds in Metal-Metal Bonded Complexes
less straightforward
Fe2(CO)9
[p-Cp)Cr(CO)3]2
Co2(CO)8
(2 isomers)
p. 20
d6 Octahedral
maximum of 6 coordinate
eg
M+
M+
Free ion
spherical
Δo
six point charges
spherically distributed
t2g
octahedral
ligand field
M+
M+
Free ion
spherical
t2
Δt
four point charges
spherically distributed
e
tetrahedral
ligand field
p. 21
Picture of Octahedral Complex
L
L
M
L
Various representations
L
(ignore “s orbital”
s orbital
L
L
lower case letters for
orbital
dz2, dx2-y2 (e2g)
(destabilized)
spherical
field of 6
charges
10Dq or Δo
Oh
dxy, dxz, dyz (t2g)
(stabilized)
p. 22
The five d-orbitals form a set of two bonding molecular orbitals (eg set
with the dz2 and the dx2-y2), and a set of three non-bonding orbitals
(t2g set with the dxy, dxz, and the dyz orbitals).
L
eg orbitals point at ligands (antibonding)
appropriate symmetry for s-bonds to ligands
dxy, dxz, dyz
s-bonds will be six d2sp3 hybrids
All are non-bonding
ndz2, ndx2-y2, (n+1)s, (n+1)px,py,pz
L
t2gLorbital set left as non-bonding
t2g set
L
dz
L
L
2
L
L
L
eg set
L
L
dx2-y2
bonding
L
L
L
p. 23
p. 24
Molecular Orbitals
metal-based
orbitals
__
__
4p __
__
__ __
__
4s
__
3d
__
__
__
__
__
__
Anti-Bonding
MO's
a1g*
__
eg*
o
__
__
non-bonding
t1u*
__
__ __
__
orbitals
e.g., [Co(NH3)6]3+ (18 e–)
e.g., W(Me)6 (12 e–)
eg
__ __
__
t2g
__
__
__
__
__
__
Standard MO diagram for
Octahedral ML6 complexes
six
with s-donor ligands
ligand
t1u
Bonding
MO's
a1g
Case I
Electron-configuration unrelated to 18–-Rule
1st Row-Complexes with “weak ligands”
Do small or relatively small, eg* only weakly antibonding
No restriction on # of d-electrons –– 12 to 22 electrons
p. 25
Case II Compounds which follow rule insofar as they
p. 26
never exceed the 18-e– rule
• Metal in high oxidation state
Do is large(r) (for a given ligand)
radius is small –-> ligands approach closely ––> stronger bonding
• 2nd or 3rd Row Metal - 4d, 5d
Do is large(er) (for a given ligand); d-orbitals larger, more diffuse.
Complex
dn
Total e– Complex
ZrF62–
ZrF73–
Zr(C2O4)44–
WCl6
WCl6–
WCl62–
TcF62–
0
0
0
0
1
2
3
12
14
16
12
13
14
15
dn
OsCl62–
W(CN)83–
W(CN)64–
PtF6
PtF6–
PtF62–
PtCl42–
Less than 18 e–, but rarely exceed 18 e–
Total e–
4
1
2
4
5
6
8
16
17
18
16
17
18
16
p. 27
Similar Result if ligands are high in Spectrochemical Series
e.g., CN– Do is larger
V(CN)63–
Cr(CN)63–
Mn(CN)63–
Fe(CN)63–
Fe(CN)63–
Co(CN)63–
d2
d3
d4
d5
d6
d6
Less than or equal to 6 d-electrons
eg* not occupied
however Co(II) d7 ––> Co(CN)53–
Ni(II) d8 ––> Ni(CN)42– and
Ni(CN)53–
Can have less than maximum # of non-bonding (t2g) electrons, because they
are nonbonding. Addition or removal of e– has little effect on complex stability
p. 28
Do can get (or is) very small with p-donor ligands
F– example (could be Cl–, H2O, OH–, etc.)
a) Filled p-orbitals are the only orbitals
capable of p-interactions
i) 1 lone pair used in s-bonding
ii) Other lone pairs p-bond
b) The filled p-orbitals are lower in
energy than the metal t2g set
c) Bonding Interaction
i. 3 new bonding MO’s filled by
Fluorine electrons
ii. 3 new antibonding MO’s form t2g*
set contain d-electrons
iii. Do is decreased (weak field)
d) Ligand to metal (L M) p-bonding
i. Weak field, p-donors: F, Cl, H2O
ii. Favors high spin complexes
p. 29
Metal
Orbital
s
T1u
A1g
Eg
T2g
4p
4s
Molecular
Orbitals
Ligand
Orbitals
focus on
this part
only
Δo
eg
(σ*)
t2g (π*)
both sets of d
orbitals are
driven ↑ in
energy due to
lower lying
ligand orbitals
T1g,T2g
3d
t2g
(π)
eg (σ)
A1g
T1u,T2u
π-orbitals
px, py
T1u σ-orbital
E pz
g
p. 30
Have discussed s-donor and p-donor – now p-acceptor
antibonding
eg (σ*)
eg (σ*)
eg (σ*)
Δo
t2g (π)
M-L bonding
Δo
t2g (n.b.)
non-bonding
σ-donor
π-acceptor
largest separation
between sets of d-orbitals
intermediate
separation
Δo
t2g (π*)
both are
antibonding
π-donor
smallest separation
Metal Orbitals
Molecular Orbitals
(only consider the d
orbitals – 4s and 4p
orbitals not included
in the analysis)
Ligand Orbitals
t2g (π*)
T1g, T2g
T1u, T2u
eg (σ* M-L)
Eg
T2g
Mo(CO)6
p. 31
CASE III
π* orbitals on CO
L high in spectrochemical series:
(6 x 2 each orthogonal)
CO, NO, CN–, PR3, CNR
p-acid ligands – p-acceptors
Can form strong p-bonds
18 e– rule followed rigorously
Δo
4d
t2g (π)
σ orbitals on CO
(6 x 1 each)
A1g
Orbitals on M used in such p-bonding
T1u
are just those which are non-bonding
Eg
eg (σ M-L)
Result: Increase in Do
Imperative to not
Have electrons in eg* orbitals
Want to maximize occupation of t2g
because they are stabilizing
p. 32
p. 33
Implications of 18e– Rule for Complexes with p-accepting ligands
In octahedral geometry almost always have 6 d-electrons
12 electrons from ligands
Other cases: # d-electrons and coordination # complementary
• Coordination # exactly determined by electron-configuration and vice-versa
BrMn(CO)5 (d ?)
(see previous notes)
I2Fe(CO)4 (d ?)
Fe(CO)5 (d ?)
All 18-electron
Ni(PF3)4 (d ?)
When M has odd electron ––––> metal-metal bond (often bridging CO’s)
Mn2(CO)10
Co2(CO)8
Some 17 electron species known: V(CO)6 d 5
Mo(CO)2(diphos)2]+ d 5
See MO diagram: Want to fill stable MO’s’ there is a large gap to LUMO
p. 34
Major Exception: d 8 square-planar complexes
As one goes across periodic table, d and p orbital energy
Level splitting gets larger – hard to use p orbitals for s-bonding
Common to have 4-coordinate SP complexes – dsp2 hybridization
dx2-y2
Which d-orbitals?
eg
Common for:
dxy
Δo
Rh(I), Ir(I)
Pd(II), Pt(II)
dz 2
t2g
dxz
dyz (degenerate )
ML6
ML4
Rationalize d-orbital splittings
look at d-orbital pictures/axes
p. 35
p. 36
Again, examples of complexes:
dn
C.N.
Coord. Geom.
Example(s)
d10
4
Td
Ni(CO)4, Cu(py)41+
d10
3
Trig.planar
d10
2
Linear
d8
5
TBP
d8
4
(square) planar
d4
7
capped octahedral
d2
8
d0
9
sq. antiprism
Pt(PPh3)3
(PPh3)AuX, Cu(py)2+
Fe(PF3)5
Rh(PPh3)2(CO)Cl (trans)
Mo(CO)5X2
ReH5(PMePh2)3, Mo(CN)84–
D3h symmetry
tricapped trig. prism
[ReH9]2–
p. 37
LIGANDS in Organometallic Chemistry:
Ligands,
charge,
coordination # (i.e., denticity)
X
SnCl3
H (hydride)
Ar
RC(O) (acyl)
R3E (E = P, As, Sb, N)
CO
RNC (isonitrile, isocyanide)
R 2N
N2
R2C (cabenoid, carbene)
C3H5– (p-allyl)
p-C5H5 (p-Cp)
p-C3H3 (cyclopropenium, +)
ArN2+ (diazonium)
R 2P
C2H4 (olefin, alkene)
C4H4 (cyclobutadiene)
benzene (arenes)
CH3 (alkyl, perfluoroalkyl)
R2C2 (acetylene)
CH=CH-CH2– (s-allyl)
p-C7H7 (tropylium)
O (O-atom; oxide)
NO (nitrosyl)
p. 38
Carbon Monoxide – exceedingly important ligand
CO-derivatives known for all transition metals
Structurally interesting, important industrially, catalytic Rxs
Source of pure metal:
Ni (Mond); Fe contaminated with Cu, purify via Fe(CO)5
Fe & Ni only metals that directly react with CO
Source of oxygen in organics: RC(O)H, RC(O)OH, esters
Processes: hydroformylation, MeOH ––> acetic acid
double insertion into olefins, hydroquinone synthesis (acetylene + CO;
Ru catalyst), acrylic acid synthesis (acetylene, CO, Ni catalyst)
Fischer Tropsch Rx: CO + H2 ––> ––> CnH2n+2 + H2O
Most of these involve CO “insertion”
p. 39
Metal-Carbonyl Synthesis:
Reduction of available (in our O2-environment) metal salts,
e.g., MX2, M’X3, other (e.g., carbonates)
M-carbonyls generally in low-valent oxidation states
––––> “Reductive Carbonylation”
Reductants: CO itself ( ––> CO2), H2, Na-dithionite
Some Reactions: WMe6 + xs CO –––> W(CO)6 +
3 Me2CO
NiO + H2 (400 °C) + CO ––> Ni(CO)4
Re2O7 + xs CO ––> (OC)5Re–Re(CO)5 + 7 CO2
Cl– acceptor/reductant
RhCl3 + CO + pressure + (Cu, Ag, Cd, Zn)
–––> Rh4(CO)12 or Rh6(CO)16
Structures Possible: X-ray diffraction, Infrared spectroscopy
Ni(CO)4
Td
2058 cm–1
Fe(CO)5
M(CO)6
D3h
2013, 2034 cm–1
Oh
2000 cm–1
p. 40
H3B–CO = 2164 cm–1
no backbonding possible
13C
NMR spectroscopy of M-CO fragments: 180 – 250 ppm
Useful to use 13C enriched carbon monoxide
Can be useful to observed “coupling” to other spin active nuclei,
e.g., 103Rh or 13P
Metal-Carbonyl Structures (cont.):
Polynuclear Metal-Carbonyls
p. 41
p. 42
p. 43
p. 44
p. 45
The backbonding between the metal and the CO ligand,
where the metal donates electron density to the CO ligand
forms a dynamic synergism between the metal and ligand,
which gives unusual stability to these compounds.
Dynamic synergism bonding
–
Valence Bond formalism:
+
O:
M=C=O
:
M–C
O
:
C
p. 46
C–O stretching frequencies, n(C-O)
Put more electron density on metal
– by charge
– by ligands which cannot p-accept
Remaining CO’s have to take up the charge (e–-density) on the metal
See effects on n(C-O).
Ni(CO)4
2057 cm–1
–––––––>
Mn(dien)(CO)3+
Cr(dien)(CO)3
[Co(CO)4]–
Fe(CO)42–
1886 cm–1
1786 cm–1
–––––––> more –ve charge
~ 2020, 1900 cm–1
~1900, 1760 cm–1 (dien not p-acceptor)